The invention, in some embodiments, relates to the field of electron beam emission and more particularly, but not exclusively, to ferroelectric emitters suitable for the emission of electron beams. The invention, in some embodiments, also relates to the field of millimeter waves, and more particularly, but not exclusively, to gyrotrons.
Ferroelectric (FE) emitters have been investigated as a cold electron source for many applications including electron guns. After long period of scientific discussion regarding the emission mechanism, several experimental devices were demonstrated, and it was proven that the FE emitter can be integrated into microwave tubes [refs. 2-7]. Recent achievements extend the use of such emitters to S-band relativistic magnetrons [ref. 8] and 95 GHz gyrotrons [ref. 9]. Depending on the implementation, FE emitters may have one or more advantages including: FE emitters are cold emitters, FE emitters can withstand relatively high currents, have a relatively short (immediate) turn on time, need no conditioning, require modest vacuum to operate, and are relatively inexpensive.
While thermionic emitters can emit long pulses and even continuous beams, plasma emitters such as FE emitters are limited to short-pulse operation [ref. 10]. Some of the factors which limit the duration of the pulses include the gap closure, and the plasma relaxation time that limits the pulse repetition frequency (PRF). The FE emission is a plasma-assisted effect. When an FE emitter is operated in an electron tube, surface plasma is ignited on a front electrode on the distal (front) side of the emitter and electrons are drawn towards the anode. Thus, an FE emitter is limited to short pulses (typically 100-300 ns). Pulse duration, PRF, and possible duty cycle of an electron tube are all determined by the emitter and limit the electron tube performance.
Emitter lifetime is another limiting factor for ferroelectric emitters. Although FE emitters have an infinite shelf lifetime and do not need refreshing when not operative, during emitter operation generated surface plasma tends to damage the emitter surface and gradually degrades emitter performance. Lifetimes of FE emitters have been studied [refs. 11-13] where the emitters were operated in different PRF's in the range of 1 Hz-1 kHz.
Research to prolong the pulse duration of electron beams generated in tubes having FE emitters has been done. Early attempts are reported in the work of Advani et al. [ref. 14] where a 5 microsecond single pulse is achieved from an 11.4 cm diameter annular ferroelectric emitter. This emitter was designed for a gyrotron but it was not implemented in an FE tube, and no radiation was obtained.
Prolonging of pulse duration in different plasma emitters, based on explosive emission, was reported by Engelko [refs. 10] where multipoint ignition was used to overcome the plasma limitation, generating a 30 microsecond current pulse length. This demonstration included an electron gun, but radiation from an electron tube was not reported. Engleko's method was later implemented by Gleizer et al. [ref. 15] with FE emitters, obtaining single pulses of ˜6 microsecond, reporting an electron beam, but without generating radiation.
Radiation from an FE tube has been reported by Hadas et al. [ref. 8], where an S-Band magnetron with an FE emitter was compared to the same tube with an explosive emission emitter. The use of the ferroelectric emitter extended the duration of the radiated pulse by ˜30% to 100 ns, and increased the microwave radiation power by ˜10%. It is clearly determined in the experiment that the FE emitter is ˜30% more efficient than an explosive emission emitter in the tested tube. In other studies demonstrating the integration of ferroelectric emitter in electron tubes in a gyrotron [refs. 3, 4], a PRF of 3 MHz and duty cycle of up to 50% was measured with 150 ns pulses. However, FE emitter tubes with long pulses were not reported.